Investigating the Birth of Magnetars Through the Analysis of High Energy Neutrinos
2009. 8. 25


Some stars die violent deaths in magnificent explosions known as supernovae. This is the fate of stars whose masses fall in a particular range, far beyond the mass of our sun. While most of the material of the progenitor star is expelled in this violent process, there remains a small fraction that forms a highly dense ``remnant," which, if the mass of the progenitor star is not too great, will come to form a neutron star, consisting mainly of neutrons. It is believed that some of these remnants become so-called magnetars, neutron stars that possess extremely strong magnetic fields, which have been estimated to be as strong as $1015$ G. To this time, there have been eighteen bodies identified as magnetars, but the source of the extreme magnetic fields they possess is yet unknown. This is regarded as one of the great puzzles of present-day astronomy. Kohta Murase, a fifth year graduate student and JSPS Special Research Fellow in the Yukawa Institute for Theoretical Physics, is working with Professors Peter Meszaros of Pennsylvania State University and Bing Zhang of the University of Nevada to solve this puzzle. They have pointed out that if newly formed magnetars can be understood as very powerful particle accelerators, then it is possible that important information concerning the origin of their magnetic fields can be obtained by studying high energy neutrinos impinging upon Earth.

As one possibility for the mechanism causing the formation of the magnetic field possessed by a magnetar, the so-called dynamo mechanism has been proposed. It is believed that if a newly formed neutron star exhibits very fast rotation about its axis, with a period on the order of milliseconds, a dynamo mechanism can act to create a strong magnetic field, with the result being a magnetar. It would be expected that such a rapidly rotating magnetar would accelerate particles in its vicinity to very high energies, and thus it is thought that magnetars could be the source of cosmic rays observed on Earth with energies as high as $1020$ eV. Murase, Meszaros and Zhang have shown that, under such conditions, through the interaction of cosmic ray protons with cold nucleons (debris from the supernova) and thermal radiation in the neighborhood of the magnetar, a peculiar neutrino flux characteristic of rapidly rotating magnetars would be created. It is possible that this neutrino signal could be detected with large-volume neutrino detectors, such as IceCube. The IceCube Observatory consists of an array of detectors covering a volume of one cubic kilometer embedded in the Antarctic ice sheet. This facility is scheduled to reach its maximum operational capacity by 2011. While some of the interesting problems that could be addressed in the proposed studies include determining whether rapidly rotating magnetars are indeed formed through the dynamo mechanism and whether they actually do act as particle accelerators, as has been conjectured, in fact, the theoretical aspects of a number of related phenomena, such as the acceleration mechanism itself, are yet unclear. Although the research conducted by Murase, Meszaros and Zhang does not necessarily support the dynamo hypothesis for magnetar formation nor the conjecture that magnetars are the source of high-energy cosmic rays observed on Earth, they have pointed out one example of the potential utility of high energy neutrino detectors in elucidating unexplained phenomena and investigating the validity of theoretical models, including these conjectures. Their work on magnetars was published in Physical Review D, and it was presented in Nature Physics as a Highlight Article.